if its a 42, its 8mm. if its a 3, its 308. imnot positive, but im pretty sure the mg-1 was 308 too.
either way, your not going to get a mg3 drum on a 42 or a 1.
the drum attaches to teh feed tray. im not sure if you can change out JUST the feed tray or not, or if its even worth the effort.
idk if you can order the parts in from the us, if so, heres an mg3 feed tray with the propper mount to attach that mg3 drum:
MG3 Feed Tray, MG42 .308 Feed Tray, RTG Gun Parts